On the first 6 tests of his language class, Michael got an average score of 85. What does he need on the next test to have an overall average of 87?
Explanation: Let his score on the next test be $x$ The sum of all of his scores is then $6 \cdot 85 + x$ The same sum must also be equal to $7 \cdot 87$ Solve: $x = 7 \cdot 87 - 6 \cdot 85 = 99$.